The present invention relates to a mass spectrograph.
It is known that a mass spectrograph is an apparatus which produces ions from a substance to be analysed and then sorts these ions in accordance with the ratio M/e of their mass to their charge. This ratio makes it possible to very accurately determine the mass of each of the ions formed and count the number of ions of each species making it possible to establish the composition of the substance.
A mass spectrograph generally comprises an ion source, an ion analyser and a detector. The ion sources are either pulsed sources or continuous sources. The analysers may be of different types, so that a distinction is made between spectrographs with a magnetic analyser in which the analyser functions as a spatial separator for the ion species and spectrographs with an electrostatic analyser, generally associated with a magnetic analyser acting as a spatial separator. This ion analyser may also comprise a free space which is not subject to any field and which acts as a time separator for the ions. In this case, the spectrograph is generally described as being of the type with "separation in the time of flight".
In a magnetic analyser, the ions from the same object slot of the ion source and which have the same kinetic energy travel in circular trajectories, whose radii of curvature are in accordance with the formula: ##STR1## In this formula, m designates the mass of 1 ion, v its velocity, e its charge and B the induction of the magnetic field to which the said ion is subject. Thus, the ion species are spatially separated.
In a time of flight analyser, all the ions initially have the same kinetic energy and they travel in the same trajectory x in a time t corresponding to formula: EQU t=X/v
in which v is the velocity of the ions.
This velocity is expressed by the formula: ##EQU1## in which E.sub.c designates the kinetic energy of a considered ion.
In this type of analyser, the ion species are separated in time in accordance with the formula t=k.sqroot.m, k designating a constant. Thus, a time of flight mass spectrograph can only function with a pulsed ion source. Thus, time t.sub.o at which the ions depart must be defined with a very great time resolution.
The mass spectrographs may also be classified in accordance with the types of detectors used. The type of detector is generally dependent on the type of analyser. In a mass spectrograph with a magnetic analyser or electrostatic analyser, coupled to a magnetic analyser, it is preferable to use a panoramic collection detector in order to increase the analytical potential of the spectrograph. Thus, in this case, it is necessary for the ionic optical system to have a large planar image surface formed from all the image points of each of the ion species from the same object slot.
In a time of flight mass spectrograph, the image point of the object slot is unique. Therefore, in this type of spectrograph, the detector used must have a very high response speed and at the same time an ion collecting surface of reduced dimensions. The photographic plate is the best known panoramic detector adapted to the spatial separation analysers. The photographic detector simultaneously collects all the ion species and integrates in time different signals collected. Its dynamics, which is the ratio of the maximum signal detected to the minimum signal detected, is of the order of 10.sup.3 to 10.sup.4. Its detection limit is 10.sup.6 ions/mm.sup.2 and per detectable line. Its spatial resolution is of the order of a few micrometers. Although this detector has a good spatial resolution and a good sensitivity, it has the disadvantage of production and utilization difficulties. A considerable amount of time is required for developing, recording and the calibration curves of the photographic plates.
It is known to replace the photographic plate by a plurality of photomultipliers, each of which is sensitive to a single beam of ions of given mass. However, this system is heavy and cumbersome and in practice does not permit the separation of adjacent ion masses.
To obviate these disadvantages, it is known to replace the panoramic photographic detector or the multiple photomultiplier system by an ion-electron-photon converter applied to analysis by spatial separation. This converter is designed in such a way that it retains the spatial resolution of the mass spectrum collected on the focal surface of the ionic optical system. It comprises for example an electron amplifier microchannel plate in which the ion-electron conversion is performed directly when an ion strikes the inner wall of a channel. This plate has several thousand small channels, whose diameter is approximately 12 microns. These channels are juxtaposed and issue into a scintillator constituted by an aluminised phosphorescent layer permitting the conversion of the electrons from the channels into photons. The photons constituting the brightness information are transported by a certain number of light guides or optical fibres, which all converge towards a spectrum reading window. This spectrum must be read by means of a Vidicon sensing camera. This system is complicated and difficult to operate. Moreover, it requires a continuous collection of the spectrum to permit reading by sensing. It has a great sensitivity and retains a certain spatial resolution permitting the separation of the different masses to be detected and measured. However, it has the disadvantage of offering a surface with blind joints between each plate channel to the incident ion beams. Consequently, the conversion surface is discontinuous and there is a loss of spatial resolution and analytical information. Moreover, in the case of this type of plate there is a certain lack of information on the ion-electron conversion characteristics for different species. Therefore, the accuracy of the measurements is uncertain.
Another known spectrograph which uses a panoramic detector permitting analysis by spatial separation, as hereinbefore, uses an ion-electron-photon conversion. The ions are converted into electrons by means of a system of taut electrical wires or tapes on which the incident ion beams produce the emission of electrons. These electrons then strike individual scintillators which produce photons. These photons are channelled into individual light guides, such as optical fibres and then lead to an individual photomultiplier for each optical fibre. A spectrograph of this type, as hereinbefore, has a discontinuous conversion surface. There is therefore a loss of spatial resolution in the detection of ions. This spectrograph also has problems as a result of a large number of photomultipliers, particularly with regard to the overall dimensions, the quality of the vacuum obtained and the regulation of the operating temperature.
In order to eliminate this discontinuity in the spatial resolution of the spectrograph detectors, it is known to construct a conversion electrode with a continuous structure. This electrode is generally brought to negative potentials and the ions strike it without being previously accelerated in a defined space. This negative potential makes it possible to limit the capture of the ions and to eliminate rebound phenomena during the impact of said ions on the conversion electrode. The secondary electrons which are emitted as a result of the impacts are channelled by an electrical field perpendicular to the plane of incidence of the ion beams. These electrons are then collected on a photographic plate after passing through a diaphragm at earth potential. Although this spectrograph permits a continuous spatial resolution, it does not permit the elimination of the use of a photographic plate.
Thus, as can be gathered from the statements made hereinbefore, there is no known spectrograph which enables time of flight measurements to be made and which at the same time has a panoramic ion detector with a continuous structure, associated with a pulsed ion source.